This invention relates to a method for determining the state of charge of lead-acid rechargeable batteries by estimating their no-load voltage during active operation in a vehicle.
It is desirable to determine the present state of charge of a rechargeable battery for numerous applications. If the present state of charge at a specific time is known, then the change in the state of charge can be deduced by continuous measurement of the current flowing, and by integrating it. However, the validity of the present measured value deteriorates to an ever greater extent due to the measurement uncertainty linked to this method, so that the state of charge reading must be reset from time to time using a different method.
In the case of a lead-acid rechargeable battery, assessment of the present no-load voltage of the rechargeable battery is highly suitable for this purpose since it is directly linked to the state of charge (as discharging continues, sulfuric acid is consumed from the electrolyte, and the no-load voltage of the lead-acid rechargeable battery is directly dependent on the acid density). However, to do this, the rechargeable battery is operated without interruption for at certain time, during which time the current flowing must be less than a very low threshold value. This is because the lead-acid rechargeable battery produces its no-load voltage only in the (virtually) unloaded state, while its voltage in some cases differs greatly from this when charging or discharging currents are flowing.
Depending on previous history, even in the unloaded state, it often still takes a long time for the internal state of the rechargeable battery to reach its equilibrium value, and for the voltage to reach its no-load value. Even with modern rechargeable batteries, the period of phases without any current flowing required after previous charging phases and at low temperatures is very long, in some cases from several hours up to a number of days.
Thus, in operating modes without such long phases with no current flowing, the method of measuring the no-load voltage for determining the state of charge is in general suitable only to a limited extent. One example of this is the starter rechargeable battery in a motor vehicle where, often, only stationary times of a few hours or, for example, when used in taxis, virtually no stationary times at all occur. At room temperature and above, the present no-load voltage which would be obtained if the current were switched off briefly is itself a good measure of the real no-load voltage of the rechargeable battery.
DE-19643012 A1 specifies a method in which no-load voltage values Uo from the zero crossings of the current flowing through the rechargeable battery are used to drive the voltage regulation of the generator. This method places relatively stringent demands on the measurement electronics. The flowing current generally passes through the zero point very quickly, making interpolation necessary, which can be achieved only with difficulty due to the non-linearities of the current/voltage curve at low currents.
Thus, it would be advantageous to provide a method which allows a conclusion to be drawn about the present no-load voltage of the rechargeable battery, as well as an estimate to be made of the no-load voltage while the rechargeable battery is on load, without artificially interrupting operation.
The invention relates to a method for determining the state of charge of lead-acid rechargeable batteries including a) substantially simultaneously measuring measured-value pairs (Ui, Ii) of the rechargeable battery voltage and current flowing at time ti over time interval dt, b) selecting a group of the measured-value pairs (Ui, Ii) for which only a discharge current flowed in the last time interval dt, c) varying parameters Uo, R and C such that a residual sum of squares between values Ui given by formula (1):
Ui=Uoxe2x88x92R*Ii+1/C ∫Idtxe2x80x83xe2x80x83(1)
and measured values U(ti) is minimized, wherein Uo represents no-load voltage, R represents resistance and C represents capacitance, and d) calculating the state of charge of the rechargeable battery from the no-load voltage obtained from formula (1).